Arctic
sea-ice cover will diminish rapidly under global warming, but its rate of
retreat in boreal winter shows large intermodel differences across the models
involved in Phase 5 of the Coupled Model Intercomparison Project (CMIP5). But
when a model simulates a larger sea-ice decline, how does the circulation
outside the Arctic change?

A
new study conducted by Prof. Noel Keenlyside (University of Bergen) and
colleagues sought to address this question by first applying singular value
decomposition (SVD) to the projection of winter Arctic sea ice and mean sea
level pressure (SLP) over Eurasia [2069–98 in the RCP8.5 run (the most severe
future global warming scenario) minus 1971–2000 in the historical run] across
11 CMIP5 models. Importantly,
the power of using SVD analysis here is in quantifying the largest
covariability between the model uncertainties of these projections, and to
depict their spatial patterns. This is better than using the relatively simpler
methods of composite or correlation analysis, which first require an index
(say, sea-ice change in the Arctic) to be defined.

The
dominant SVD mode has an explained variance of 70.5%, and it corresponds to a larger
pan-Arctic sea-ice decline. Normally, intense cold air masses sink in the Arctic
and move equatorward near the surface. This drives the polar cell that
exchanges air masses with the mid-latitudes. When a model simulates a larger pan-Arctic
sea-ice decline, the Arctic becomes warmer and less cold air sinking in the
polar region. The associated polar cell is weaker and its equatorward side has
anomalous sinking motion. In Eurasia, the mean SLP response shows an increase
in the Urals–Siberia region and near Iceland, while it decreases in the
Mediterranean. The anomalous SLP responses over the Euro-Atlantic
region project onto the negative North Atlantic Oscillation–like pattern, which
is the dominant mode of large-scale circulation in the Euro-Atlantic region. Therefore, a more
accurate winter Arctic sea-ice projection could be useful for constraining
projections of winter Eurasian climate.